Liquid scintillation counters are commonly used to measure the count rate or activity of samples containing low energy beta particles or corresponding particles emitting radionuclides such as tritium, iodine-125, carbon-14, sulphur-35, calcium-40 and chromium-51.
The range of the low energy beta particles in the sample is in general few tens of micrometers at the most. As a consequence, the sample to be measured has to be placed in direct contact with a scintillation medium by dissolving or suspending the sample within the liquid scintillation medium in a container so that the emitted beta particles can interact with the molecules of the liquid scintillation medium, which comprises a solvent or solvents and a solute or solutes which constitutes a small weight percent of the solution. In this interaction process most of the kinetic energy of the interacted beta particle is absorbed by the solvent and then transferred to the solute which emits scintillation photons, whose amount is proportional to the energy of the interacted beta particle. These scintillation photons are detected by two photomultiplier tubes, operating in coincidence, produce electric pulses. The sum pulse height is proportional to the energy of the interacted beta particle.
When measuring sample activities with liquid scintillation counters, the basic problem is the reduction of counting efficiency due to the quenching of the sample, which can be classified in two main types: a chemical quench and a color quench. The chemical quench is a phenomenon in which the solution formed by the sample and the scintillation medium contains some impurities, which reduce the efficiency of the counting system to detect the emitted beta particles by absorbing them. The color quench is a phenomenon in which the solution formed by the sample and the scintillation medium contains some impurities which absorb produced scintillation photons. The consequence of this reduces also the counting efficiency.
It is known in liquid scintillation counting that the reduction of counting efficiency due to the quenching of the sample can be corrected by means of a quench curve which describes the relationship between the counting efficiency and amount of the quench of the sample. Normally liquid scintillation counters are provided with one detector and the quench curve is obtained by measuring a number of standards with identical activities but different quench levels in the detector. Difficulties are be encountered when a liquid scintillation counter is provided with a plurality of detectors: all standards should be measured in each detector due to the variations in energy and efficiency responses of the detectors. This is a time consuming procedure.